Efficacy of UV-attenuation by organic particulate UV filters in sunscreens depends on particle size

Sunscreens often are semi-fluid systems such as creams, lotions, gels or sprays, aimed for the application on human skin for the protection against solar UV-radiation. In most cases they contain an aqueous and an oil phase and are prepared as emulsions. Organic UV-filter molecules can be dissolved in one of the phases of the emulsions. In addition, particulate UV-filtering substances, either inorganic or organic, may be suspended. Organic particulate UV-absorbers have become important actives in modern sunscreens. The specific extinction is a measure of the UV-attenuating efficacy of UV-filtering materials. In this work, a theoretical method is developed which can be used to describe the dependence of the specific extinction on particle size of particulate organic UV-absorbers. To this end, the extinction coefficient of the material dissolved in a suitable solvent is required, which represents a maximum limit for the performance of the particulate material. Using the specific extinction as a measure for efficacy, it is shown that the smaller the particles, the better is their protective effect. For the case of dispersions of very small UV filter particles with sizes approaching molecular dimensions, the specific extinction of the respective molecules in solution is approached, the extinction in solution representing a maximal limit. Good agreement with experimental data is demonstrated for the examples of two organic particulate UV filters, methylene bis-benzotriazolyl tetramethylbutylphenol and tris biphenyl triazine. The method can be used to simulate the particle size dependence of the UV attenuating efficacy of organic particulate UV filters.